Imperial College Rock Library

Glossary: Cumulate

    A cumulate is a rock formed by accumulation of cumulate crystals by settling within a body of magma. Cumulates can be found in intrusive bodies and thick lava flows.
 
Cumulates can be subdivided on the basis of the proportion of cumulate crystals relative to crystals formed from the trapped intercumulus liquid. Adcumulates comprise >95% cumulate crystals. Mesocumulates have 95-85% cumulate crystals. Orthocumulates consist of <85% cumulus crystals.
 
Poikilitic textures are common within orthocumulates. Secondary growth of cumulate phases from intercumulus liquid and reaction replacement of crystals with the liquid are common processes in cumulate piles. Intercumulus liquid may also be intruded through cumulate piles by compaction.
 
Thick sequences of cumulates are preserved in layered intrusions. Layering in such intrusions can be rhythmic, exhibiting repeated layers that contain abundant mafic minerals at their base and feldspars at the top. Cryptic layering in which the composition of the cumulate changes through the pile, and phase layering, where one or more phases are absent from certain layers, can also occur. Cumulates can also exhibit depositional structures indicating deposition within a current, and preferred orientation of elongate phases, formed by settling and later compaction.
 
Many coarse-grained igneous rocks, in particular gabbros, peridotites and dunites are cumulates. Xenoliths derived from magma chamber cumulates are simply termed cumulate xenoliths.

Related Terms

crystal fractionation, poikilitic, intercumulus

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